The present invention relates to a method which is used for measuring a three dimensional shape of a fine pattern formed on a semiconductor device, such as a semiconductor memory or an integrated circuit.
SEMs (scanning electron microscope) are used for measuring fine patterns that are formed on semiconductor devices. The SEM obtains an electron beam image of a sample by detecting secondary electrons and reflected electrons that are generated when an electron beam is irradiated onto the sample. The most popular SEM used currently in semiconductor processing is called a critical dimension SEM, which measures a sample mainly by using a secondary electron beam image.
FIG. 2 shows the relationship between a cross sectional shape and a secondary electron beam image of a sample. The greater the slope of a surface of the sample, the greater will be the strength of secondary electrons emitted therefrom, so that, as shown in FIG. 2, an image is obtained having bright portions (hereinafter called bright bands) corresponding to side portions (slope portions) of the sample pattern and dark portions corresponding to plane portions of the sample pattern. With the bright bands, the dimensions d1 and d2 are measured to obtain a bottom size and top size of the sample, respectively. However, three dimensional information, such as the height H of the sample and the slope angle θ of the side surface cannot be obtained.
In semiconductor processing, the critical dimension SEM has been conventionally used for optimizing the conditions of a manufacturing machine, such as an aligner and etcher, or for monitoring process fluctuation. However, with refining of the patterns, three dimensional shapes of the samples need to be measured in various cases, wherein the critical dimension SEM is not always useful.
Examples of related technology for measuring cross sectional shapes are as follows.
(1) After a wafer is cut or FIB-processed, a cut surface of the wafer is observed using an electron microscope.
(2) The cross sectional shapes are observed using an AFM (Atom Force Microscope).
(3) The cross sectional shapes are observed using scatterometry. However, in these methods, the following problems are encountered.
In the method (1), it takes a long time to prepare for observation of the cross sections. Additionally, the cut or FIB-processed wafers tend to become contaminated, and, thus, they cannot be completed as products. As a result, this method cannot be used for process fluctuation monitoring in a quantity production process.
The method (2) does not take a longer time than that in the method (1) to observe the cross sections; however, the AFM has a low throughput, which is about ⅓ of that of the popular critical dimension SEM, and it cannot be used to measure all patterns because of restriction of the chip shapes. Consequently, as it is near-meaningless, critical points cannot be measured in the monitoring of process fluctuation in which measurement of three dimensional shapes is required.
Recently, the scatterometry method (3) has received attention, because it can be operated at high speed, and it can be used to measure cross sectional shapes non-destructively. Using the fact that spectral distribution of scattered light from a sample changes depending on the material and cross sectional shape of the sample, the scatterometry method matches the spectral distribution of the actually-measured sample to the spectral distribution library of various cross sectional shaped models previously produced using offline simulations, thereby to indirectly measure the cross-sectional shape of the sample (see FIG. 3). In principle, any pattern shape can be produced. However, current computers cannot generate a library including variations of all patterns. In the present condition, only lines and space patterns uniformly repeated in one direction are measurable. As a result, the scatterometry method is used only for measuring test-specific patterns that are formed on a wafer, and it cannot be used to measure arbitrary patterns (for example, critical points for process fluctuation).
Technology related to the present invention is disclosed in JP-A No. 141544/1991, JP-A No. 342942/1992, and JP-A No. 506217/2002. However, the technology disclosed in these publications have the following problems. The critical dimension SEM, which is popular in semiconductor processing, can measure plane shapes by use of electron beam images of arbitrary patterns, but it cannot be used to measure three dimensional shapes. The scatterometry method can measure three dimensional shapes, but the sample patterns are limited to lines and spaces. Therefore, the scatterometry method can be used to measure only those shapes which conform to the test patterns produced for measurement.